The following list of references is herein cited as part of the background information:    1. A. Zhamu, et al., “Graphene nanocomposites for electrochemical cell electrodes,” U.S. patent application Ser. No. 12/220,651 (Jul. 28, 2008); US Pub. No. 20100021819 (Jan. 28, 2010).    2. Guo, P. et al. “Electrochemical performance of graphene nanosheets as anode material for lithium-ion batteries,” Electrochem. Comm. 11, 1320-1324 (2009).    3. Bhardwaj, T., et al., “Enhanced Electrochemical Lithium Storage by Graphene Nanoribbons,” J. Am. Chem. Soc. 132, 12556-12558 (2010).    4. Lian, P. et al. “Large reversible capacity of high quality graphene sheets as an anode material for lithium ion batteries,” Electrochim. Acta 55, 3909-3914 (2010).    5. Wu, Z.-S., “Doped graphene sheets as anode materials with superhigh rate and large capacity for lithium ion batteries,” ACS Nano 5, 5463-5471 (2011).    6. A. Zhamu, et al., “Mixed nano-filament electrode materials for lithium ion batteries,” US Pub. No. 20090176159 (Jul. 9, 2009).    7. A. Zhamu, et al., “Hybrid nano-filament cathode compositions for lithium metal or lithium ion batteries,” US Pub. No. 20090186276 (Jul. 23, 2009).    8. J. Shi, et al., “Conductive nanocomposite-based electrodes for lithium batteries,” US Pub. No. 20090305135 (Dec. 10, 2009).    9. A. Zhamu, et al., “Nano graphene reinforced nanocomposite particles for lithium battery electrodes,” US Pub. No. 20100143798 (Jun. 10, 2010).    10. C. G. Liu, et al., “Lithium Super-battery with a Functionalized Nano Graphene Cathode,” U.S. patent application Ser. No. 12/806,679 (Aug. 19, 2010).    11. C. G. Liu, et al, “Lithium Super-battery with a Functionalized Disordered Carbon Cathode,” U.S. patent application Ser. No. 12/924,211 (Sep. 23, 2010).    12. Aruna Zhamu, C. G. Liu, David Neff, and Bor Z. Jang, “Surface-Controlled Lithium Ion Exchanging Energy Storage Device,” U.S. patent application Ser. No. 12/928,927 (Dec. 23, 2010).    13. Aruna Zhamu, C. G. Liu, David Neff, Z. Yu, and Bor Z. Jang, “Partially and Fully Surface-Enabled Metal Ion-Exchanging Battery Device,” U.S. patent application Ser. No. 12/930,294 (Jan. 3, 2011).    14. Aruna Zhamu, Chen-guang Liu, X. Q. Wang, and Bor Z. Jang, “Surface-Mediated Lithium Ion-Exchanging Energy Storage Device,” U.S. patent application Ser. No. 13/199,450 (Aug. 30, 2011).    15. Aruna Zhamu, Chen-guang Liu, and Bor Z. Jang, “Partially Surface-Mediated Lithium Ion-Exchanging Cells and Method of Operating Same,” U.S. patent application Ser. No. 13/199,713 (Sep. 7, 2011).    16. Bor Z. Jang, C. G. Liu, D. Neff, Z. Yu, Ming C. Wang, W. Xiong, and A. Zhamu, “Graphene Surface-Enabled Lithium Ion-Exchanging Cells: Next-Generation High-Power Energy Storage Devices,” Nano Letters, 2011, 11 (9), pp 3785-3791.
Graphene is a common building block for most of the carbonaceous and graphitic materials, including graphite, carbon nanotube, carbon nano-fiber, graphite fiber, carbon black, activated carbon, meso-phase carbon, coke, soft carbon, and hard carbon. For instance, graphite is composed of multiple crystallites that are essentially stacks of relatively large graphene sheets (50 nm—several μm in width), while carbon black is made up of small graphene sheets or aromatic rings (lateral dimensions of 10-50 nm) connected by disordered carbon. A carbon nano-tube (CNT) is composed of one or multiple sheets of graphene rolled into a tubular shape, but the CNT and graphene are distinct in morphology, structure, and composition having vastly different properties. The CNT (one-dimensional tube) and graphene (two-dimensional sheet) are two distinct classes of materials. A carbon/graphite fiber contains flat and/or curved graphene sheets as the main constituent structural element. All of these materials can be chemically or physically unzipped or exfoliated, and then separated to obtain single-layer or multi-layer graphene sheets.
Most of the commercially available Li-ion cells make use of carbon- or graphite-based anodes, which have several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g of graphite), slow Li intercalation (due to low solid-state diffusion coefficients of Li in graphite) resulting in a long recharge time, inability to deliver high pulse power, and necessity to use lithiated cathodes (e.g. lithium cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathodes have a relatively low specific capacity (typically <200 mAh/g) and also rely upon extremely slow Li diffusion in the solid state. These factors have contributed to the two major shortcomings of today's Li-ion batteries—a low energy density (typically 120-180 Wh/kgcell) and low power density (<1 kW/kgcell).
Theories set forth earlier predicted that the anode capacity limit of 372 mAh/g could be overcome provided the inter-graphene spacing in graphite can be increased from 0.335 nm to the range of 0.4-0.8 nm. Higher Li storage capacities were observed with carbon materials containing graphene sheets dispersed in a disordered carbon matrix, but these anode materials suffer from large capacity irreversibility. The possibility of storing Li at the edges of internal graphene planes or internal imperfections in the bulk of a graphite particle has been suggested, but these mechanisms in the carbon anode are also inherently irreversible. Furthermore, all of these proposed approaches to lifting the Li storage capacity limit still require storing Li in the bulk of graphite/carbon particles, which necessitates solid-state diffusion of Li during battery discharge and charge.
Our research group reported an approach to liberating graphitic and carbonaceous materials from the aforementioned constraints, leading to the discovery of an anode active material with a higher specific capacity [Ref. 1 above]. This approach entails preparing a composite composition comprising multiple solid particles, wherein (a) a solid particle is composed of graphene platelets dispersed in or bonded by a first matrix or binder material; (b) the graphene platelets have a length or width in the range of 10 nm to 10 μm; (c) the multiple solid particles are bonded by a second binder material; and (d) the first or second binder material is selected from a polymer, polymeric carbon, amorphous carbon, metal, glass, ceramic, oxide, or organic material. For a lithium ion battery anode application, the first binder or matrix material is preferably amorphous carbon or polymeric carbon. Such a composite composition provides a high anode capacity and good cycling response. For a supercapacitor electrode application, the solid particles preferably have meso-scale pores therein to accommodate electrolyte. In this report, there was no teaching about using graphene as a cathode active material.
Subsequently, several research groups [Ref. 2-5] have also used isolated graphene sheets as an anode active material, directly exposing some graphene surfaces to liquid electrolyte. An initial lithium storage capacity of typically 600-1,500 mAh/g was reported, but these graphene-based anodes suffer from large first-cycle irreversibility (up to 50%, likely due to the formation of solid-electrolyte interface, SEI) and rapid capacity decay during subsequent cycles as well.
Our research group also reported [Ref. 6-9] the use of graphene platelets as a supporting substrate for a cathode active material. Graphene platelets assist in the formation of a 3-D network of conducting paths for the cathode active material. In these reports, there was no teaching about using graphene as a cathode active material; graphene had not been recognized as a cathode active material then (and even up to this point in time). Graphene was used to improve the electrical conductivity of the cathode since all the then known cathode active materials (e.g. inorganic metal oxide and lithium iron phosphate) were electrically non-conducting.
For instance, Ref. [6] provides a mixed nano-filament composition comprising: (a) an aggregate of nanometer-scaled, electrically conductive filaments (e.g. graphene and CNT) that are substantially interconnected, intersected, or percolated to form a porous, electrically conductive filament network; and (b) Multiple nanometer-scaled, electro-active filaments comprising an electro-active material capable of absorbing and desorbing lithium ions wherein the electro-active filaments have a diameter or thickness less than 500 nm. The electro-active filaments (e.g., Si nanowires) and the electrically conductive filaments (e.g., graphene and carbon nano fibers) are mixed to form a mat-, web-, or porous paper-like structure in which at least an electro-active filament is in electrical contact with at least an electrically conductive filament. There was no teaching, implicitly or explicitly, about using graphene as a cathode active material.
Ref. [7] provides a hybrid nano-filament composition comprising (a) an aggregate of nanometer-scaled, electrically conductive filaments that are substantially interconnected, intersected, or percolated to form a porous, electrically conductive filament network, wherein the filaments have a length and a diameter or thickness with the diameter or thickness being less than 500 nm; and (b) micron- or nanometer-scaled coating that is deposited on a surface of the filaments, wherein the coating comprises a cathode active material capable of absorbing and desorbing lithium ions. It is this coating that is the cathode active material in the hybrid composition. There was no teaching, implicitly or explicitly, about using graphene as a cathode active material.
Ref. [8] provides a nanocomposite-based lithium battery electrode comprising: (a) A porous aggregate of electrically conductive nano-filaments that are substantially interconnected, intersected, physically contacted, or chemically bonded to form a three-dimensional network of electron-conducting paths; and (b) Sub-micron or nanometer-scale electro-active particles that are bonded to a surface of the nano-filaments with a conductive binder material, wherein the particles comprise an electro-active material capable of absorbing and desorbing lithium ions and wherein the electro-active material content is no less than 25% by weight based on the total weight of the particles, the binder material, and the filaments. The bonded particles are the electrode active material, not the supporting graphene sheets. There was no teaching, implicitly or explicitly, about using graphene as a cathode active material.
Ref. [9] provides a solid nanocomposite particle composition comprising: (A) an electrode active material in a form of fine particles, rods, wires, fibers, or tubes with a dimension smaller than 1 μm; (B) nano graphene platelets (NGPs); and (C) a protective matrix material reinforced by the NGPs; wherein the graphene platelets and the electrode active material are dispersed in the matrix material. Graphene was used to reinforce or improve the structural integrity of a protective carbon matrix that serves to protect an electrode active material. There was no teaching, implicitly or explicitly, about using graphene as a cathode active material.
Most recently, we proceeded to go beyond the mindset of using graphene either as an anode active material or as a cathode supporting material by investigating the feasibility of implementing graphene as a cathode active material. This is of great scientific and technological significance since the common cathode materials, such as lithium cobalt oxide and lithium iron phosphate, have relatively low specific capacities (typically <<200 mAh/g) and, hence, a strong need exists for a higher-capacity cathode. Further, a material is a good anode active material for a lithium-ion cell, the same material is usually not considered to be a viable cathode active material for a lithium-ion cell from the electrochemical potential perspective. We have defied this expectation with the discovery that graphene can be used as a high-capacity and high-power cathode material in a surface-mediated cell or SMC [Ref. 10-18].
There are two types of SMCs: partially surface-mediated cells (p-SMC, also referred to as lithium super-batteries) and fully surface-mediated cells (f-SMC). Both types of SMCs contain the following components: (a) an anode containing an anode current collector (such as copper foil) in a p-SMC, or an anode current collector plus an anode active material in an f-SMC; (b) a cathode containing a cathode current collector and a cathode active material (e.g. graphene or disordered carbon) having a high specific surface area (preferably >100 m2/g); (c) a porous separator separating the anode and the cathode, soaked with an electrolyte (preferably liquid or gel electrolyte); and (d) a lithium source disposed in an anode or a cathode (or both) and in direct contact with the electrolyte.
In a fully surface-mediated cell, f-SMC, as illustrated in FIG. 2, both the cathode active material and the anode active material are porous, having large amounts of graphene surfaces in direct contact with liquid electrolyte. These electrolyte-wetted surfaces are ready to interact with nearby lithium ions dissolved therein, enabling fast and direct adsorption of lithium ions on graphene surfaces and/or redox reaction between lithium ions and surface functional groups, thereby removing the need for solid-state diffusion or intercalation. When the SMC cell is made, particles or foil of lithium metal are implemented at the anode (FIG. 2(A)), which are ionized during the first discharge cycle, supplying a large amount of lithium ions. These ions migrate to the nano-structured cathode through liquid electrolyte, entering the pores and reaching the surfaces in the interior of the cathode without having to undergo solid-state intercalation (FIG. 2(B)). When the cell is re-charged, a massive flux of lithium ions are quickly released from the large amounts of cathode surfaces, migrating into the anode zone. The large surface areas of the nano-structured anode enable concurrent and high-rate deposition of lithium ions (FIG. 2(C)), re-establishing an electrochemical potential difference between the lithium-decorated anode and the cathode.
A particularly useful nano-structured electrode material is nano graphene platelet (NGP), which refers to either a single-layer graphene sheet or multi-layer graphene pletelet. A single-layer graphene sheet is a 2-D hexagon lattice of carbon atoms covalently bonded along two plane directions. We have studied a broad array of graphene materials for electrode uses: pristine graphene, graphene oxide, chemically or thermally reduced graphene oxide, graphene fluoride, chemically modified graphene, hydrogenated graphene, nitrogenated graphene, doped graphene. In all cases, both single-layer and multi-layer graphene materials were prepared from natural graphite, petroleum or coal tar pitch-derived artificial graphite, other types of artificial graphite, etc. These micro-structures can be exfoliated to allow for easy separation or isolation of graphene sheets from one another.
These highly conducting materials, when used as a cathode active material, can have a functional group that is capable of rapidly and reversibly forming a redox reaction with lithium ions. This is one possible way of capturing and storing lithium directly on a graphene surface (including edge). This is however not a preferred or desired way of storing lithium. We have also discovered that the benzene ring centers of graphene sheets are highly effective and stable sites for capturing and storing lithium atoms, even in the absence of a lithium-capturing functional group.
In a p-SMC, the anode comprises a current collector and a lithium foil alone (as a lithium source), without an anode active material to capture and store lithium ions/atoms. Lithium has to deposit onto the front surface of an anode current collector alone (e.g. copper foil) when the battery is re-charged.
The features and advantages of SMCs that differentiate the SMC from conventional lithium-ion batteries (LIB), supercapacitors, and lithium-ion capacitors (LIC) are summarized below:                (A) In an SMC, lithium ions are exchanged between anode surfaces and cathode surfaces, not bulk or interior of an electrode active material:                    a. The conventional LIB stores lithium in the interior of an anode active material (e.g. graphite particles) in a charged state (e.g. FIG. 1(C)) and the interior of a cathode active material in a discharged state (FIG. 1(D)). During the discharge and charge cycles of a LIB, lithium ions must diffuse into and out of the bulk of a cathode active material, such as lithium cobalt oxide (LiCoO2) and lithium iron phosphate (LiFePO4). Lithium ions must also diffuse in and out of the inter-planar spaces in a graphite crystal serving as an anode active material. The lithium insertion or extraction procedures at both the cathode and the anode are very slow, resulting in a low power density and requiring a long re-charge time.            b. When in a charged state, a LIC also stores lithium in the interior of graphite anode particles (FIG. 1(E)), thus requiring a long re-charge time as well. During discharge, lithium ions must also diffuse out of the interior of graphite particles, thereby compromising the power density. The lithium ions (cations Li+) and their counter-ions (e.g. anions PF6−) are randomly dispersed in the liquid electrolyte when the LIC is in a discharged state (FIG. 1(F)). In contrast, the lithium ions are captured by graphene surfaces (e.g. at centers of benzene rings of a graphene sheet as illustrated in FIG. 2(D)) when an SMC is in a discharged state. Lithium is deposited on the surface of an anode (anode current collector and/or anode active material) when the SMC is in a charged state. Relatively few lithium ions stay in the liquid electrolyte.            c. When in a charged state, a symmetric supercapacitor (EDLC) stores their cations near a surface (but not at the surface) of an anode active material (e.g. activated carbon, AC) and stores their counter-ions near a surface (but not at the surface) of a cathode active material (e.g., AC), as illustrated in FIG. 1(A). When the EDLC is discharged, both the cations and their counter-ions are re-dispersed randomly in the liquid electrolyte, further away from the AC surfaces (FIG. 1(B)). In other words, neither the cations nor the anions are exchanged between the anode surface and the cathode surface.            d. For a supercapacitor exhibiting a pseudo-capacitance or redox effect, either the cation or the anion form a redox pair with an electrode active material (e.g. polyaniline or manganese oxide coated on AC surfaces) when the supercapacitor is in a charged state. However, when the supercapacitor is discharged, both the cations and their counter-ions are re-dispersed randomly in the liquid electrolyte, away from the AC surfaces. Neither the cations nor the anions are exchanged between the anode surface and the cathode surface. In contrast, in a SMC, the cations (Li+) are captured by cathode surfaces (e.g. graphene benzene ring centers) when the SMC is in the discharged state. It is also the cations (Li+) that are captured by surfaces of an anode current collector and/or anode active material) when the SMC is in the discharged state. In other words, the lithium ions are shuttled between the anode surfaces and the cathode surfaces.            e. An SMC operates on the exchange of lithium ions between the surfaces of an anode (anode current collector and/or anode active material) and a cathode (cathode active material). The cathode in a SMC has (a) benzene ring centers on a graphene plane to capture and release lithium; (b) functional groups (e.g. attached at the edge or basal plane surfaces of a graphene sheet) that readily and reversibly form a redox reaction with a lithium ion from a lithium-containing electrolyte; and (c) surface defects to trap and release lithium during discharge and charge. Unless the cathode active material (e.g. graphene, CNT, or disordered carbon) is heavily functionalized, mechanism (b) does not significantly contribute to the lithium storage capacity.                            When the SMC is discharged, lithium ions are released from the surfaces of an anode (surfaces of an anode current collector and/or surfaces of an anode active material, such as graphene). These lithium ions do not get randomly dispersed in the electrolyte. Instead, these lithium ions swim through liquid electrolyte and get captured by the surfaces of a cathode active material. These lithium ions are stored at the benzene ring centers, trapped at surface defects, or captured by surface/edge-borne functional groups. Very few lithium ions remain in the liquid electrolyte phase.                When the SMC is re-charged, massive lithium ions are released from the surfaces of a cathode active material having a high specific surface area. Under the influence of an electric field generated by an outside battery charger, lithium ions are driven to swim through liquid electrolyte and get captured by anode surfaces, or are simply electrochemically plated onto anode surfaces.                                                (B) In a discharged state of a SMC, a great amount of lithium atoms are captured on the massive surfaces of a cathode active material. These lithium ions in a discharged SMC are not dispersed or dissolved in the liquid electrolyte, and are not part of the electrolyte. Therefore, the solubility limit of lithium ions and/or their counter-ions does not become a limiting factor for the amount of lithium that can be captured at the cathode side. It is the specific surface area at the cathode that dictates the lithium storage capacity of an SMC provided there is a correspondingly large amount of available lithium atoms at the lithium source prior to the first discharge/charge.        (C) During the discharge of an SMC, lithium ions coming from the anode side through a separator only have to diffuse in the liquid electrolyte residing in the cathode to reach a surface/edge of a graphene plane. These lithium ions do not need to diffuse into or out of the volume (interior) of a solid particle. Since no diffusion-limited intercalation is involved at the cathode, this process is fast and can occur in seconds. Hence, this is a totally new class of energy storage device that exhibits unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range. This device has exceeded the best of both battery and supercapacitor worlds.        (D) In an f-SMC, the energy storage device operates on lithium ion exchange between the cathode and the anode. Both the cathode and the anode (not just the cathode) have a lithium-capturing or lithium-storing surface and both electrodes (not just the cathode) obviate the need to engage in solid-state diffusion. Both the anode and the cathode have large amounts of surface areas to allow lithium ions to deposit thereon simultaneously, enabling dramatically higher charge and discharge rates and higher power densities.                    The uniform dispersion of these surfaces of a nano-structured material (e.g. graphene, CNT, disordered carbon, nano-wire, and nano-fiber) at the anode also provides a more uniform electric field in the electrode in which lithium can more uniformly deposit without forming a dendrite. Such a nano-structure eliminates the potential formation of dendrites, which was the most serious problem in conventional lithium metal batteries (commonly used in 1980s and early 1990s before being replaced by lithium-ion batteries).                        (E) A SMC typically has an open-circuit voltage of >1.0 volts (most typically >1.5 volts) and can operate up to 4.5 volts for lithium salt-based organic electrolyte. Using an identical electrolyte, a corresponding EDLC or symmetric supercapacitor has an open-circuit voltage of essentially 0 volts and can only operate up to 2.7 volts. Also using an identical electrolyte, a LIC operates between 2.2 volts and 3.8 volts. These are additional manifestations of the notion that the SMC is fundamentally different and patently distinct from both an EDLC and a LIC.        
The amount of lithium stored in the lithium source when a SMC is made dictates the amount of lithium ions that can be exchanged between an anode and a cathode. This, in turn, dictates the energy density of the SMC.
In these co-pending patent applications [Ref. 10-15] we used graphene as a cathode active material for a SMC cell, wherein the anode contains only a current collector or a current collector and an anode active material having high surfaces on which lithium can be electrochemically deposited. The anode active material (e.g. graphene or activated carbon) in a SMC does not involve lithium intercalation and de-intercalation. In the instant application, graphene is used as a cathode active material for a lithium-ion cell that contains a high-capacity anode active material (e.g. Si, Sn, or SnO2) and/or a high-rate capable anode active material (e.g. nano-scaled Mn3O4 particles). These anode active materials (e.g. Si, Sn, SnO2, Mn3O4, and lithium titanate) in the presently invented lithium-ion cell operate on lithium intercalation and de-intercalation. These combinations lead to several unexpected yet highly significant results. Experimental evidence indicates that the electrochemical behaviors of these Li-ion cells and the SMC cells are vastly different and fundamentally distinct.